The present invention relates to drilling in earth formations. More particularly, the invention relates to a rotation mechanism which employs a hydraulic motor to rotate a drill string during drilling. The drilling application may be for drilling water, oil, ground control-like piling operations, blast hole drilling, etc.
Basic drilling methods include “percussive” drilling and “rotary” drilling. The choice of drilling method is mainly dependent upon the physical and geological properties of the earth formation to be drilled. Hard rock formations generally require percussive drilling, while soft or non-consolidated rock may be suited to non-percussive rotary drilling.
In percussive drilling, percussion energy is generated by a reciprocating piston. With each piston impact from the piston, tungsten carbide buttons in the drill bit penetrate the rock surface. After each impact, the drill string is rotated to turn the drill bit to a new position as that the buttons strike fresh rock surfaces.
There are two types of percussive drilling, namely top hammer percussive drilling, wherein the percussion energy is applied by a piston to an upper end of the drill string, and down-the-hole percussive (DTH) drilling wherein the percussive energy is applied by a piston to a lower portion of the drill string, just above the bit. Top hammer drilling is generally used for drilling relatively small-diameter holes, e.g., 3–4 inches, whereas DTH drilling is generally used for drilling slightly larger-diameter holes, e.g., 4–6 inches.
Rotary drilling does not use percussion, but compensates by having increased feed force and rotation torque. The torque causes the bit to rotate, while the feed force holds the bit firmly against the ground. The combination of rotary torque and feed force enables the bit to produce chips by crushing and cutting. Rotary drilling is generally used for drilling holes greater than six inches in diameter.
A typical mobile drilling rig for performing blast-hole drilling (i.e., percussive or rotary) is depicted in FIG. 1. Blast-hole drilling is employed in the extraction of rock products and minerals from surface mines and quarries. A blast-hole drill produces holes according to a predetermined pattern and depth. The holes are charged with explosive, and the rock/minerals are blasted and broken for simplified recovery. The drilling rig comprises a mobile carriage 12 on which a mast 14 is supported. The mast carries a rotary head 16 which is capable of rotating a drill string 18 to which a drill bit 20 is mounted. The rotary head 16 can be raised and lowered by a hydraulically driven up-down feed system, e.g., a chain mechanism, to enable pipes to be removed from, or added to, the drill string.
A conventional rotary head 16, depicted in FIGS. 2–3, includes a housing 22, a hydraulic motor 24 mounted on a top side of the housing, and a rotation transmission mechanism carried within the housing for transmitting rotation from the motor to the drill string. The rotation transmission mechanism includes a speed reduction gear system 28 connected to the motor, and a bull shaft 30 connected to the gear system for outputting rotation to the drill string.
The gear system can be of any suitable configuration for performing a speed-reducing function. The bull shaft 30 is suitably splined to a bull gear 32 of the gear system to be rotated thereby about a vertical axis. An upper drill pipe of the drill string would be connected to a lower end 34 of the bull shaft.
The motor 24 is typically a piston-type hydraulic motor mounted on a top side of the housing 22. Hydraulic cylinders and roller chains, or cables (not shown) function to raise and lower the rotary head, which is secured to the mast with adjustable wear pieces (guide shoes).
As the drill string advances, during percussive or rotary drilling operations, it alternately encounters harder and softer rock formations, as well as cracks and voids in the rock formations. Thus, the resistance to rotation of the drill string is frequently changing, causing the drill string rotation to accelerate and decelerate. As a result of flexibility in the drill pipes, the mast 14 and the undercarriage 12, the repeated acceleration/deceleration of the drill string tends to produce heavy vibrations which can lead to premature wear and failure of the parts being vibrated, as well as creating discomfort for the operating personnel. To deal with that problem, it is often necessary to reduce the speed of rotation and drilling in order to limit the vibration magnitude, but that undesirably reduces the rate of penetration of the drill string through the earth formation.
While those problems occur in both percussive and rotary drilling methods, they are especially evident in rotary drilling where the torque and rotary speeds are much greater than in percussive drilling and thus result in stronger vibrations.
In all drilling applications, the frequent acceleration and de-acceleration cause premature drill bit and drill string damage as well as premature structural failures on the drill rig.
It would, therefore, be desirable to minimize vibrations during drilling (rotary or percussive) without having to appreciably reduce the rate of penetration. It would also be desirable to achieve that result in a relatively economical way.